Japanese Laid-Open Patent Publication No. 2010-9321 describes a touch type input device including a touch panel that is touched to operate virtual buttons shown on a display. In the touch type input device, for example, a user touches an operation surface of the touch panel to select one of a plurality of functional items shown on the display. This allows the user to view the desired interface on the display or operate an onboard device. Such touching of the touch panel includes, in addition to a simple touch of the operation surface, swiping or flicking of the operation surface in which a finger is moved along the operation surface in a single direction. For example, by swiping or flicking the operation surface, the interface shown on the display is scrolled.
Such a touch panel may implement projected capacitive technology. A projected capacitance type touch panel includes drive electrodes and sensor electrodes that are arranged in a grid-shaped pattern. Capacitors are formed at the intersecting points of the drive electrodes and the sensor electrodes. Touching of the touch panel is detected from the capacitance of each capacitor. A mutual capacitance type touch panel detects a touched position based on changes in the capacitance of each capacitor. The mutual capacitance type touch panel is advantageous in that a number of touch positions may be simultaneously detected.
A conductive foreign matter such as a water droplet or a coin may contact or collect on the operation surface of the touch panel. The contact of such a foreign matter needs to be distinguished from a finger that touches the operation surface. In a mutual capacitance type touch panel, when a finger contacts the operation surface, the capacitance of each capacitor in the area touched by the finger changes from a reference value (zero), which corresponds to a condition in which there is no object contacting the operation surface, toward a first polarity side (e.g., positive). In contrast, when a foreign matter contacts the operation surface, the capacitance of each capacitor in the area that is in contact with the foreign matter changes toward a second polarity side (e.g., negative), and the capacitance of each capacitor in the area around the foreign matter changes toward the first polarity side. Japanese Patent No. 4994489 describes a touch type input device that takes this into consideration. The touch type input device computes the distribution of the amount of change in capacitance of the capacitors. When the peak value of the changed amount distribution is less than or equal to a predetermined negative threshold, the touch type input device determines that a foreign matter is contacting the operation surface.
When a conductive foreign matter remains in contact with the operation surface of the touch panel, the capacitance of each capacitor remains changed from the reference value. This hinders the detection of a touch. Thus, the touch type input device of Japanese Patent No. 4994489 shows a message on a display to prompt the user to remove the foreign matter from the touch panel.
However, in the touch type input device described above, the conductive foreign matter has to be removed from the operation surface to allow for detection of the touching of the operation surface. This is inconvenient, especially, when the touch type input device is installed in a vehicle and a foreign matter cannot be readily removed from the operation surface.